`fixUnencryptedSize()` asked the encryption module for the unencrypted
block size before calling `begin()`. The module only learns the encoding
of the file (binary or legacy base64) from the header in `begin()`, so
the block size of binary encoded files was used for legacy base64 files.
This stored an inflated unencrypted size for legacy files, which then
breaks downloads: the announced content length is too large and the
signature check of the last block fails with "Bad Signature".
Fixes#63662
Assisted-by: ClaudeCode:claude-opus-5-5
Signed-off-by: Ferdinand Thiessen <opensource@fthiessen.de>